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Chemie

Figurę t. The [CHB_nRsXt| carborane ions.

stron gest isolablc (Lewis-free) Bronsted acids presently known.

Two reactivity considerations have led us to believe that carborane acids must have uncommon strength. First is their stabilization of the benzenium ion in salts such as (C_f,H_r| |CHB||Me₅Br_h]. The strongest known neat oxyacid. HFSOj, (whose Hammett acidity function //„=-15.1) is not strong enough to fully protonatc benzene.¹¹ yet carborane acids do so readily at the one-equivalent level in dilute soiution.¹'¹ Second. we have recently shown that carborane anions stabilizc len-butyl cation in dichloromethane or liquid SO_: soiution, as well as in isolable crystalline salts.¹' This means that carborane acids are strong enough to shift the equili-brium of Equation(l) completely to the right hand side. lf this were not the casc, deprotonation of tm-butyl cation to give /jo-butene and subsequent decomposition by cationic oligomerization would occur rapidly.

^H><\    HA, ₄

C—CH, I H(Cartorane) =^s=s=- ~^Chj I Caibo'ane (1) HjC    HjC

Since mixed Lewis/Bronsted superacid media such as ”Magic Acid“ (HFSOj/SbFO having H„ somcwhcrc in the rangę -17 to -27 are used to stabilize leri-butyl cation,^{1 s} it is apparent that carborane acids have intrinsic Bronsted acid-ities comparable to those only found previously in Lewis/ Bronsted acid muctures,

The aciditiesof carborane acids ca nnot be measured in the conventional manner of an //, Hammett acidity function because carborane acids are solids not liquids, They sublime under vacuum. or melt at atmospherie pressurc. at temper-atures well above 150°C- Gas-phasc acidities have been calculated for carborane acids, which rank them the strongest of any known isolable acid.^,ł[ but it is presently not possible to translate gas-phasc AG data into measures of condcnsed-phase acidity.

The need to compare the strengths of solid acids with liquid acids therefore led us to obtain a measurc of acidity in dilute soiution. conditions where most acid catalysis is carried out. The chosen solvent is liquid SO. because of its Iow basicity. relatively high dielcctric constant for sobation of ions, and casc of drying. The chosen method is adapted from that developed by Farcaijiu and Ghenriu.¹⁷' It is based on the "CNMR spectroscopy Chemical shift difference (Ad) between the C„ and Cp carbon atoms of mesityl oxidc which inereases with inereasing protonation as F.quation (2) is shifted to the right hand side.

H.C ' \ C,=^£	H /	H*	^ha. r Ct- c.
HjC	C |:		HjC ' C
	0		OH

^: 'C NMR spectroscopy data for 0.15 M Solutions of various acids and O.IOm mesityl oxidc were obtained at room temperaturę and are given in Table l.The choice of oxyacids

Tobie i: Acid-strength ranking from protonation of mesityl oxide in l>quid SO,.

Ac»d	Ad (”C) (ppm)
H(CHB,,CI„)	84.o±ai	M
H(CHB- .HiCIJ	8J 8 rO.l	W
H(CHBtlHłBrł)	83.8 ±0.1	W
H(CHB-,HsI4)	83.3 ±0.1	M
FSO,H	73.8 ±0.5	—15.1
CF,SO,H	72.9±0.4	— 14.1
HN(SO,CF,),	72.0±0.4	H
H,SO.	64.3 ± 3.1^w	—12.1
Unprotonated mesityl oxide	32.4 ±0.1

[a] Hę valucs unavailable because acids are solids. notliquids. [b] Incom-plete miscibility of H.SO, in liquid SO, leads to higher error limits and possible underestimate of Ad.

for comparison to carborane acids is limited by their avail-ability in pure form and the need for miscibility or dissolution in liquid SO.. Nevertheless. it is immediately evident from their high Chemical shift valucs that. as a class. carborane acids are stronger than comentional oxyacids. They outrank fluorosulfuric acid. the strongest known pure Bronsted acid on the H₀ acidity scalę (—15.1), as well as triflic acid (//„= -14.1). A strict correlation between Ad \alues in liquid SOj and the H₀ valucs of neat acids is not cxpcctcd because acidity is a soKation-depcndent phenomenon that changes as the medium changes. Nesertheless, for the limited data set available. the ranking of acid strength by Ad values fołlows the //„order (Table 1).

It is also evident from the data of Table I that whereas oxyacids only partially protonate mesityl oxide. carborane acids are strong enough to move Equation (2) complctely to the right hand side, that is. their acidities are Ieveled. The Ad value maximiz.es at d = 84.0 ppm. Though close to the limits of discrimination. the hcxa-iodo carborane acid. [HfCHBuHdJ) with a value of d = 833 ppm. appears to be perceptibly weaker than its chloro and bromo counterparts.

To discriminate between the acidities of the different carborane acids. we have developed a ncw, qualitative ranking of acidity based on the rN-H frequencies of the ammonium salts of their conjugate base anions. The ranking is based on the influence of the anion A on the N-H bond in a contact ion pair (Figurę 2).

Figurę z Ammonium salt contact ion pair.

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